Various gaseous fluorine-containing compounds are utilized in manufacturing processes that plasma-etch silicon-type materials in order to fabricate semiconductor devices. A major use of tetrafluoromethane (CF4 or FC-14) is for plasma etching during semiconductor device fabrication. Plasma etchants interact with the surface of an integrated circuit wafer, modifying it so as to lay down the electrical pathways and provide for the surface functionalities that define the integrated surface. A major use of nitrogen trifluoride (NF3) is as a “chemical vapor deposition” (CVD) chamber cleaning gas in semiconductor device manufacture. CVD chamber cleaning gases are used to form plasmas, which interact with the internal surfaces of semiconductor fabrication equipment to remove the various deposits that accumulate over time.
Perfluorinated chemicals such as CF4 and NF3 that are used in semiconductor manufacturing applications as etchant or cleaning gases are more commonly referred to as “electronic gases”. Electronic gases having high purity are critical for such semiconductor device manufacture applications. It has been found that even very small amounts of impurities in these gases that enter semiconductor device manufacturing tools can result in wide line width, and thus less information per device.
The desire for greater precision and consistency of the effect that compounds such as CF4 and NF3 have during integrated circuit manufacture has made extremely high purities critical for such applications. The presence of any other compounds in the CF4 or NF3 is objectionable for most of the intended uses. It should be recognized that either CF4 or NF3 might in itself be considered an impurity if present in the product stream of the other. For example, even a 1 part-per-million-molar concentration of CF4 would be considered an impurity in NF3 where that NF3 is to be used as a cleaning agent product. Similarly, even a 1 parts-per-million-molar concentration of NF3 would be considered an impurity in CF4 where that CF4 is to be used as an etchant product. Processes that enable the manufacture of CF4 or NF3 products having purities that approach 99.999 molar percent purity are desirable, but processes that provide at least 99.9999 molar percent purity for electronic gas applications are preferred. Analytical methods for gauging such low concentrations of impurities in CF4 and NF3 products are available. For example, methods for analyzing low concentrations of CF4 and other impurities in an NF3 product are disclosed in the 1995 SEMI standards, pages 149-153, SEMI C3.39.91-Standard for Nitrogen Trifluoride. Alternately, techniques for analyzing the concentration of CF4 and other impurities at low concentrations in FC-116, but which may also be applied to analysis of NF3 and CF4 products, are disclosed in “Examining Purification and Certification Strategies for High-Purity C2F6 Process Gas”, Micro Magazine, April 1998, page 35.
Conventional processes for manufacturing NF3, however, often produce CF4 as a component in the NF3 product stream. Because conventional processes are not able to separate the CF4 from the NF3 product, NF3 products containing less than about 10 ppm-molar CF4 are not available in spite of the desirability of lower concentrations of CF4 in said NF3 product.
Moreover, the presence of impurities, including but not limited to particulates, metals, moisture, and other halocarbons in the plasma etchant or cleaning gas, even when present at only the part-per-million level, increases the defect rate in the production of high-density integrated circuits. As a result, there has been increasing demand for higher purity etchant and cleaning gases, and an increasing market value for the materials having the required purity. Identification of offending components and methods for their removal consequently represent a significant aspect of preparing these gases, particularly fluorine-containing compounds, for use for such purpose.
Etchant and cleaning gases are not fully consumed by semiconductor manufacturing processes, but typically exit the integrated circuit fabrication equipment in finite concentrations. These fabrication equipment exhaust streams not only contain varying amounts of unreacted perfluorinated etchant and cleaning gases, but may also contain a variety of reaction products and air components, which include without limitation hydrogen fluoride (HF), tetrafluoroethylene (C2F4 or FC-1114), methyl fluoride (CH3F or HFC-41), trifluoromethane (CHF3 or HFC-23), chlorotrifluoromethane (CClF3 or CFC-13), nitrogen, oxygen, carbon dioxide, water, methane, ethane, propane and nitrous oxide (N2 O ). Typically, this results in a stream containing a wide range of CF4, NF3, and other fluorinated impurities in a wide range of concentrations, and this exhaust stream may also contains relatively high volume concentrations, typically greater than 50 volume %, of inert carrier gases such as air, helium or nitrogen.
Exhaust streams coming off of processes in which gases such as CF4 and NF3 are used are also frequently combined with exhaust streams from other types of semiconductor manufacturing activities. These other activities can generate a variety of waste gases in their own right, such as hexafluoroethane (C2F6 or FC-116), octafluorocyclobutane (cyclic C4F8 or FC-C318), octafluoropropane (C3F8 or FC-218), sulfur hexafluoride (SF6), pentafluoroethane (C2HF5 or HFC-125), trifluoromethane (CHF3 or HFC-23), tetrafluoroethane (C2H2F4, or HFC-134a or HFC-134) and difluoromethane (CH2F2 or HFC-32). The resulting combined exhaust stream consequently may contain a wide range of compounds and at widely varying concentrations.
Concerns over possible environmental impact of such materials and the high value-in-use of these materials has prompted a search for methods of recovering CF4 or NF3 from said exhaust streams of such processes. Conventional methods of recovering the components from such streams typically involve water washing the exhaust stream to remove the HF and HCl, then drying the stream using a variety of methods. Conventional methods for separating and recovering the fluorinated compounds from the large concentrations of inert carrier gases include use of semi-permeable membranes or adsorption of the fluorinated compounds into liquid solvents. However, a wide range of fluorinated organic and inorganic compounds typically still remain in the captured stream after such processing steps, making any CF4 or NF3 contained within unsuitable for reuse as electronic gases.
There is thus considerable interest in developing methods to capture fluorinated compounds that are present in manufacturing equipment exhaust streams, and in developing options for their disposition. A preferred disposition option is to repurify certain of the fluorinated components from these streams for reuse. Separation of several of these valuable fluorinated compounds is made difficult, however, due to the variety of fluorinated compounds that might be present in the combined exhaust gas stream from any given manufacturing site, and due to non-ideal interactions that exist between several of these compounds. For example, several of these compounds form azeotropes, azeotropic compositions, or azeotrope-like compositions with other compounds in these streams, making separation by conventional distillation at least difficult, if not impossible. The ability to separate and recover a NF3 product that is substantially free of CF4 and other fluorinated impurities, particularly where the CF4 concentration in the NF3 product is preferably less than 3, more preferably less than 1, ppm-molar, is thus of considerable commercial interest. The ability to separate and recover a CF4 product that is substantially free of fluorinated impurities is also of considerable commercial interest.
Many of the fluorinated compounds used or that are produced in semiconductor process operations are extremely close-boiling in their separated and pure states. Compounds whose selectivities approach or equal 1.0 compared to CF4 or NF3 make their separation from said CF4 or NF3 by conventional distillation difficult. Separation of such mixtures is particularly problematic where it is desired that the recovered CF4 or NF3 product be substantially free of other fluorinated compounds and where the CF4 or NF3 product needs to be recovered from a first mixture with high recovery efficiency.
U.S. Pat. No. 6,458,249, which is incorporated in its entirety as a part hereof for all purposes, provides a process for separating CF4 and NF3 from each other, and from mixtures with other materials used in the electronics industry, by distilling a mixture comprising NF3 and/or CF4 in the presence of an entraining agent, such as nitrous oxide or hydrogen chloride. The use of a nitrous oxide entrainer requires cryogenic temperatures for achieving good separation. Hydrogen chloride is a strong acid that presents waste disposal problems. There thus remains a need for an environmentally suitable and energy efficient process for the separation of compounds such as NF3 and/or CF4.